Electrochemical cells are generally classified as primary or secondary batteries. The primary classification includes disposable batteries that are discharged once, and are not recharged, such as, dry cells for flashlights. However, many primary type batteries are not discarded, and some parts are renewed or changed each time another single discharge is desired.
The secondary classification includes storage batteries or batteries that are recharged from an external source of electricity after they are discharged. These batteries are designed so that the electrochemical processes are reversible to a high degree, by reversing the direction of current through the battery, drawn from an external source.
It is known within the art to utilize various means to renew the electrolyte(s) and/or the electrode(s) of electrochemical cells, or to recharge them from an external source, but electrochemical cells which are capable of self-charging, utilizing means substantially wholly within the cell, have not heretofore been demonstrated.
Batteries with high discharge capabilities, or high current density, versus cell weight have been designed having non-aqueous molten salt electrolytes, that allow use of highly reactive electrode materials. These materials cannot be used with aqueous electrolytes because of undesirable corrosion or reaction problems.
Molten salts generally have higher electrolytic conductivities than do aqueous electrolytes. Molten salts allow use of more reactive solid metal materials and provide more options in configurations.
Discharge capabilities are conventionally expressed in specific energy units of watt hour per kilogram of electrode or cell weight, or specific power units of watts per kilogram. Discharge capabilities are expressed in amp hours of discharge over an eight hour period. For example, the most common storage battery in use: the six cell, twelve volt, lead/lead-sulfate battery, generally has an eight hour discharge rate around 320 amp hours, or a discharge rate at about 40 amps per hour per battery, or about 62/3 amps per hour per cell. Generally, this battery produces 30-50 watt hour per kilogram.
A high discharge rate cell is generally considered as having greater than 100 watt hour per kilogram of cell. If electrode weight differences are considered, for example, between the lower reactivity lead material weight of about 700 pounds per cubic foot versus the higher reactivity iron, chromium, or manganese material weights of about 500 pounds per cubic foot, specific energy differences are more apparent between a conventional storage and a high discharge rate battery.
In order to generate medium level discharge rates and have the self-rechargeable feature, it is necessary to have a low electronegativity difference between the reactants at the anode and cathode, and low equivalent weight electrodes, but a high reaction efficiency.
An electrolyte, besides having the characteristic of high electrolytic conductivity to promote high rates of electrochemical reactions for providing medium level discharge rates during operation, must also have very low electronic conductivity or low reactivity to avoid self-discharging by spontaneous chemical reactions at any significant rate during idle periods. While the battery operating temperature may be reduced below the melting point temperature of the salt during idle periods to stop any self-discharging, this time delay to return to the operating temperature for startup may be avoided by selecting compatible materials. Molten salt melting point temperatures have been reduced considerably by blending salts to form eutectic metallic molten salts and also by complexing with various compounds.
Operating an electrochemical cell at an elevated temperature creates problems with regard to suitability of available materials, seal leakage problems as related to cell interconnection corrosion and containment of the reactants, and in the case of liquid electrodes, many other problems, such as, electrode separation and containment of the reactants. These problems, however, are solved by currently available technology, including the use of high temperature microporous electrode separators.
Using known insulation designs, heat loss from the battery that is input to maintain its operating temperature will be negligible.
In order to make efficient use of electrode surface areas and electrolyte, the electrochemical cell, or battery, must be arranged so that electrochemical reactions are evenly dispersed over the total electrode surfaces. Solid electrodes in combination with a high conductivity molten salt electrolyte may allow sufficient electrode gaps so that a microporous separator may not be required to prevent localized electrical shorts due to electrode material shape change, dendriting, or high rates of electrode material migration. However, a metal is appreciably soluble in its own salts and any continuity in electronic conductivity through the electrolyte will necessitate usage of a separator.
Aqueous electrolytes generally increase the numbers of extraneous chemical reactions to maintain a chemical balance and reversible reactions, adding complexity, resistance, and power loss within the cell. Aqueous electrolytes generally limit the available electrode materials because of problems with solubility and corrosion. Also, aqueous electrolytes cause hydrogen gassing problems that may be vented which creates safety considerations, and creates electrical polarization problems on the electrodes that increases cell resistance and loss of cell performance.
The use of low reactant solid electrode materials may cause shape change, dendriting, and material shedding problems because of the special material preparation required to enhance discharge capabilities, such as, use of metal powders, sintered metals, or compound pastes to form electrodes.
Additional limitations of traditional aqueous electrolyte batteries are that the battery is affected by climatic temperature changes that change the reaction rate of the electrochemical reaction which changes the electrical output of the battery, the cells in the batteries are consumed by the electrochemical reaction over time and have to be replaced, and the used battery contains hazardous materials that have to be disposed of in accordance with environmental regulations.
These limitations when combined additionally limit the utility of an electric powered car. The capital cost of the car is increased over the life of the car because the aqueous electrolyte batteries deteriorate and need replaced. Also, the aqueous electrolyte batteries only have a limited charge and need recharged which limits the driving range of the electric car.